Improved dealing with bad pointers

@nh2 's comment in #76
> In the future I think it would be nice to observe invalid accesses in more detail over the API, e.g. to be able to write the let testProgram = "example-programs-build/bad-read-pointer" test such that you can assert that it does exactly 1 bad read() syscall.
Then people could use hatrace to test their programs like "this program shouldn't try to make any syscalls working on invalid data, and if it does, tell me which ones and what they do".